Square Body Fuel Pump vs Round & Solid State vs Mechanical Pumps

Voila what i found: my pump is fully electronic and it does not really have points. Two things struck me: the contact surface on the breaker was really dirty (crud) and the trapeze mechanism that engages and disengages the contact, in the original 3.5 turns fitting if the stem into the mechanism would barely disengage, thermal expansion making things probably worse.

I cleaned everything and refitted the diaphragm, which was perfectly flexible despite often fuelling e10, making sure I did not engage the stem in the mechanism the full 3.5 turns, but rather 2.75. I verified that manually pushing the diaphragm allowed the trapeze to work properly.

All refitted and cleaned and so far so good. Will try with hot weather in a couple if days.

Here you have a couple of pictures.

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The black square surface on the backplate of the electronics (last picture) is the contact area for the copper breaker. It was covered in soot. The trapeze really only released to contact at the very end of the diaphragm travel. Let’s hope it works.



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It was just the diaphragm. One the last picture that uploaded, you see the part that i mean.

It was installed direct on the chassis. I added some grommets. Tomorrow it will be the SC though…


This pic…

If I didn’t know, better, I would’ve thought @Shrapnel had had at it!


Back to the topic: received and installed a (relatively) inexpensive fully transistorized round body from SNG. Fits perfectly, and works perfectly up to 4500 rpm (did not try higher). Capacity, in case given by the diaphragm and solenoid must be very similar.

I will now get my square body rebuilt with a optical breaker -no more points-. The SNG one needed sanding (yes really) from new. It was part of the installation instructions.

I just installed a round body replacement pump as sourced from Moss in my '52 FHC. It has the electronic control, and was less expensive than other’s I investigated. This pump required a bracket to be fabricated to mount though. No big deal, but was not merely a bolt-in.

So now I wish to rebuild my original square body pump with electronic control, and keep it in the trunk as a spare. Is Halls the go-to for this? If so, what is their contact info?

Ps: they all use points, electronic or not. I intend to modify one with an optical obturator. The points in my full electronic square body went wrong.

Hi Lluise…if we just look at the SU pumps there are 2 types of electronic pumps…the first type kept the points but had an electronic unit added so the points only switched low current…then there was a redesign electronic unit that works by electro magnet with no points…one of our forum mrmbers @MarekH makes and sells an electronic conversion board to fit in the standard round points pump…it works great and far cheaper than the usuals sell .Steve

Burlens has the Hall Effect conversion kit and diaphragms.

Thanks for the clarification. I did not know. Mine must have been SNG (the circuit is identical to the replacement that I did get from SNG anyway). Points went bad. I found someone here https://su-benzinepompen.nl/ that apparently has created a fully contactless unit (high or low). Since my old is now a spare, I will see what he can do with his conversion.

The simplest and cheapest upgrade is to fit the points saver upgrade to stop the points from deteriorating because of the high currents going through them. It’s been sitting in the classified section all along.

kind regards

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My impression (looking at mine) is that the points glazed, rather than melted as the older pump (which I am familiar with from other applications). Edit: or were covered by a fine layer of oxide?).

I guess that some moisture, or whatever condensation, and with time they needed a fine sanding, but just for clarity I confirm that my shot pump, was electronic with low current points, and still failed at the points. Although installed by the previous owner, it must have been much younger than 4000 miles.

Another note: in case the amp for the current in a Mosfet, which I guess it is, I would not be surprised that a combination of lower current due to glazing and temperature (which is what I experienced), was the culprit of its failure.

Hi Marek,

I am slightly confused, and you are probably the best person to ask

If the electronic unit works by switching the current, I understand that, but does the full current to move the diaphragm still not need to pass through the closed points and coil ?

I closely examined several sets of points and they are eroded in a similar fashion to arc welding, and my understanding of this is it is high back EMF caused by the points opening

To alleviate this, I fitted TVS diodes, and this totally eliminated the visible spark

My understanding of this is they block & dissipate the EMF, but the full current still passes through the points

So that would be a different process than your electronic control unit (which I greatly admire)

All comments welcome, always willing to learn

Dear Tony,

the pump still draws the same current and carries on exactly as before.

The SU diagrams posted by Rob Reilly in post 10 above interpose a transistor into the circuit.

A transistor is also known as a current multiplier and it also acts as a switch. When a small current goes into the transistor base (marked “B”), it controls a much larger current passing through the other two terminals (marked “E” and “C”); when no current goes into the base, no current flows between the other two terminals. This means the circuit passes a small current through the points just like before but the transistor handles the large current for the pump.

The points damage you’ve observed (as does everyone else) is because the high currents passing directly through the points cause arcing as the current is high enough to try and jump the gap every time the points open and close. A suppressor attempts to tame this by slowing down the rate of change of current through the points, but using a transistor simply drops the points current down into the mA region, so the mechanical wear and tear is eliminated. The reverse EMF you mention is also dealt with by the SU circuit because it conducts through the diode rather than through the points. The circuit has its own (much smaller) suppressor across the points - a smaller one does the job because a smaller current is flowing.

All I have done is have the insight that circuit boards can be made non-square and have designed something that preserves your points timing, fits flush on the pump end, requires no soldering and removes the risk that the rats tail of soldering ever comes undone.

The reason SU never made such a printed circuit board is that, at the time, you couldn’t CNC cut the bell shaped hole in the circuit board to keep the old points - they opted for a much more complicated magnetic switching mechanism instead. My version costs half the price of theirs and requires no soldering.

Anyone with an old style SU pump which is at its half life should fit this next time they are doing maintenance.

kind regards


Thanks for the explanation Marek

I now understand how the electronic circuit works

From a technical and price perspective I would recommend your unit to any rebuilder as they prolong the life and add to reliability and is the optimum solution

There is a good chance the 54 yr old AU303 on my 420G are unrebuilt original.

Not many non-Jag fuel pumps last that long

It needed the points filing down due to spark erosion.

I fitted the TVS diode as I had them on hand, completely suppressed all visual spark

after examining various points under magnification it appears to me spark erosion is the major killer, ~ 50% of each surface was cratered out, it took a fair while for me to file the points smooth & level

I restored 3 this way that are suitable for my 420G, so now I have 2 fully tested spares in the boot (there is a LOT of room in a 420G boot), and a couple of older ones, including an LCS

Its possible the capacitor fitted to the units to help with spark suppression become ineffective after so many years.

Replacing the capacitor with a diode made a world of difference in spark

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